Question

In drosophila (fruit flies), eye colour is sex-linked and red eye colour is dominant to white eye
colour. In a cross between a red-eyed male and a heterozygous female, what is the probability
of producing white eyed female? ___________ [2 Marks]
8. Black fur in grasscutter (B) is dominant to brown fur (b.) Short tails (T) are dominant to long
tails (t). What fraction of the progeny of the cross BbTt x BBtt will have black fur and long
tails? _____________ [2 Marks]
9. In certain doves, grey feather is dominant to white. If a heterozygous dove is crossed with a
homozygous grey feather dove, what is the probability that the offspring will be white?
_______________ [2 Marks]
10. Hemophilia is a sex-linked recessive trait in humans. If a father and a son are both
hemophiliacs, but the mother is normal, her genotype must be _______ [2 Marks]
11. Two true-breeding stocks of guinea pigs are crossed. One parent has red eyes, short tail and
the other has white eyes, long tai; all F1 individuals have red eyes, short tail. If 1,000 F2
offspring resulted from the cross, approximately how many of them would you expect to have
red eyes, long tail? (Assume independent assortment). _________ [2 Marks]
12. A tall sheep is crossed with a short sheep, and the progeny are all intermediate in size between
the two parental sheep. This could be an example of ___________ [2 Marks]
13. A woman has six sons. What is the chance that her next child will be a daughter?
____________ [2 Marks]

Answer 1

7.

The probability of producing white eyed female is nil.

8.

9.

No white offspring observed in the next generation.

10.

If father and son are hemophilics, then mother should be a carrier. Son will get X-chromosome from mother and Y chromosome from father. Y chromosome has no genes. So whatever allele is in the X-chromosome will be expressed by the son. So mother has passed on the hemophilic allele to her son. X^HX^h is the mother's genotype.

Father and son are X^hY and X^hY both. The recessive allele 'h' is on the X chromosome which is causing hemophilia.